Question about Unique Contraint

[Thomas Polliard]

public class Account {
        @Id
        @GeneratedValue(strategy=GenerationType.AUTO)
        private Integer Id = null;
        @Unique
        private String accountName = null;
        @Basic
        private Date creationDate = null;
        @ManyToMany(cascade=CascadeType.ALL)
        private List<Host> hostList = new ArrayList<Host>();

However, in the database it actually doesn't restrict the accountName but 
rather forces a new ID to be generated with the same accountName. What page can 
I find out about creating a Unique constraint that mimics this in pseudo 
database ddl

create table Account (
        Id              int4            PRIMARY KEY,
        accountname     varchar(255)    NOT NULL UNIQUE,
        .....


Thanks for the assistance.

Thomas


PS:

http://openjpa.apache.org/builds/1.0.2/apache-openjpa-1.0.2/docs/manual/ref_guide_mapping_jpa.html#ref_guide_mapping_jpa_unique

Doesn't appear to be the proper Annotation.